U.S. Pat. No. 4,229,813 discloses a method in which at the beginning of the time measurement a liquid-filled capsule is pressed and its liquid is applied to the end of a strip of filter material printed with a dye. The liquid dissolves the dye in the strip and as a consequence of the diffusion which now occurs, the dye migrates along the strip in a predetermined time so that a user can identify how long the time measurement has been in progress from a scale applied to the front. In this arrangement it can be seen as less advantageous that the time elapsed can only be read off very inaccurately because no sharp edge of the dye is formed on the indicator strip.
Furthermore, a special combination of material to achieve a sharp front during a diffusion process used for time measurement is protected from the Japanese Patent Specification 1141976. In this case, the carrier material is agar-agar gel, the substrate or dissolving substance is water and the dye is methylene blue. Since the agar-agar gel used already contains the solvent water, the starting process appears to be relatively difficult. Furthermore, the gel variant has a large surface area which favors intensified drying out before the actual application of the time measurement.
Known from U.S. Pat. No. 4,802,255 is a toothbrush with a usage indicator wherein a dye contained in the toothbrush filaments gradually escapes with increasing use as a result of the action during cleaning of the teeth.